Optical communications systems, that is systems operating in the visible or near visible spectra, are now at an advanced stage of development. Such systems utilize glass fibers as the transmission medium. These fibers, generally having an overall cross-sectional diameter of about 125 .mu.m, are generally composed of at least two portions, a central core and a cladding layer disposed about the core. The cladding layer has an index of refraction which is less than that of the core, with a typical index variation from core to clad being in the range from about 0.01 to 0.05. Optical fibers in use may be single-mode or multimode. The former is characterized by a sufficiently small core to accommodate efficiently only the first order mode. Such single-mode optical structures may have a core diameter of about 8 .mu.m. Multimode optical fibers typically have cores which have a diameter in the range of about 50 .mu.m to 100 .mu.m.
Multimode optical fibers appeared to be on the wane but interest in them has been renewed recently, particularly for use in local area networks. The relatively large core facilitates splicing and permits more efficient energy coupling to a light energy source and to a repeater.
The introduction of many modes into or, alternatively, the generation of many modes within the optical fiber gives rise to a dispersion limitation which takes the form of a smearing due to the differing velocities of different order modes. Mode dispersion effects have been minimized by a continuous focusing structure. This structure takes the form of a fiber, the index of which is graded from a high value at the core center to a lower value at the cladding. The fundamental mode is generally confined to the highest index, corresponding to the lowest velocity region while higher order modes are confined generally to the relatively low index, corresponding to high velocity, regions.
A number of procedures have been utilized for manufacturing optical glass fibers. Most have yielded to procedures which in some way involve vapor source material. Typically, chlorides, hydrides, or other compositions of silica, as well as desired dopants, which tailor the index of refraction, are reacted with oxygen to produce deposits which directly or ultimately serve as glass source material from which optical fiber is drawn. Dopant materials include compositions with, for example, fluorine for lowering the index of refraction and germanium, titanium, aluminum, and phosphorous for increasing the index. Where the ultimate product is to be a graded multimode optical fiber, index gradation may be accomplished, for example, by altering the amount or type of dopant during deposition.
One technique for producing a lightguide fiber for use in communications is referred to as modified chemical vapor deposition (MCVD). It comprises directing a constantly moving stream of gas phase precursor reactants together with oxygen through a glass substrate tube having a generally circular cross-section. The oxygen stream carries silicon tetrachloride and dopants to produce the desired index of refraction in the finished optical fiber. The substrate glass is heated to a homogeneous reaction temperature within a moving zone of heat, also called a hot zone, that traverses constantly the length of the tube, and the consequent reaction produces doped silicon dioxide. The process involves homogeneous reactions that form particles away from the tube wall. The particles come to rest on the tube wall are fused into a continuous layer on the inner wall of the tube. For each pass of the moving hot zone, a layer of glass formation is deposited. The resulting tube is referred to as a preform tube. Homogeneously produced glass particles collect on the tube walls, and are fused into a continuous layer within the moving hot zone. With the usual heating means, there is a simultaneous heterogeneous reaction so that a glassy layer is produced within the moving hot zone by reaction at the heated wall surface. The substrate tube within which formation is taking place is continuously rotated about its own axis to enhance the uniformity of deposition about the periphery. See U.S. Pat. No. 4,217,027 which issued on Aug. 12, 1980, in the names of J. B. MacChesney and Paul B. O'Connor.
Continuous fusion within the hot zone and the resultant thickness uniformity of deposit facilitates the formation of an optical structure having a graded index of refraction. Gradients may be produced by varying the composition of the reactants with the ratio of high index-producing dopant increasing, in this instance, with successive hot zone traversals. The manufacture of a preform also includes altering the temperature and/or the flow rate during processing.
Initially, one end of the tube is supported in the headstock of a lathe and the other end is welded to an exhaust tube that is supported in the tailstock. Combustible gases are directed through a housing and nozzles of a torch assembly and toward the tube as it is turned rotatably about its longitudinal axis and as the torch assembly is moved therealong on a carriage to produce a moving hot zone. A temperature profile is produced across the hot zone which moves along on the surface of the tube, with a peak value sufficient to accomplish the desired reaction and deposition. See F. P. Partus and M. A. Saifi "Lightguide Preform Manufacture" beginning at page 39 of the Winter 1980 of the Western Electric Engineer.
During the deposition mode, the torch carriage moves slowly from the headstock of the lathe where dopants are moved into the glass tube to the tailstock where gases are exhausted. At the end of each pass from headstock to tailstock, the torch carriage is returned rapidly to the headstock for the beginning of another cycle and the deposition of another glassy layer.
Subsequent to the deposition mode, a collapse mode is used to cause the preform tube to become a solid rod-like member which is called a preform. It is this preform from which lightguide fiber is drawn. See D. H. Smithgall and D. L. Myers "Drawing Lightguide Fiber" beginning at page 49 of the hereinbefore identified Winter 1980 issue of the Western Electric Engineer. In order to collapse the preform tube, the torch assembly is moved in a number of passes from tailstock to headstock. The temperature of the moving hot zone which is higher during the collapse mode than during the deposition mode softens the tube wall and allows surface tension to cause the tube to collapse into a rod.
Preforms adequate for preparation of one or a few kilometers of optical fiber may be prepared during deposition periods of one or a few hours. These preforms are prepared by conventional processing from the deposited product to a final configuration which may be of rod shape. In usual processing, the tube which served as the deposition substrate becomes the cladding layer. It may, in accordance with the system, be composed of pure silica or of silica which has been doped to alter, generally to reduce, its index.
In the prior art MCVD, processes, a constant amount of silica is caused to be deposited in each layer during a pass of the torch. This is accomplished by controlling the concentration and flow rates of the gas phase precursor reactants. Disadvantageously, the resulting refractive index curve is characterized by a plurality of perturbations in amplitude across each layer. In a typical MCVD process-produced preform, about fifty layers are deposited. Contrasted to this, a preform which is made by some of other known technique may be made with two hundred or more passes.
The perturbations or ripples in the refractive index curve are indicative of substantial differences amplitude of the in the index of refraction across each of the deposited layers. Significant perturbations in the refractive index curve result in the optical fiber having less than an expected bandwidth as well as additional loss. It is known that as the number of passes and hence the number of layers increases, the bandwidth increases.
Although the use of substantially more passes and hence more deposited layers improves the bandwidth of the resulting optical fiber, there is a drawback. Increased numbers of passes cause the process to become increasingly less economical.
What is desired and what seemingly is not available in the prior art are methods for making a preform from which may be drawn relatively high bandwidth, low loss multimode optical fiber. Such sought-after processes and apparatus should be capable of being integrated easily into conventional MCVD processes and apparatus.